1. Field of the Invention
The present invention relates to large-diameter metal pipes joined at their ends (for example, pipes for oil pipelines or fuel pipelines of a diameter equal to or greater than 200 mm), the inner wall of which is equipped with a heat-sensitive covering. It also relates to a process for producing the joined metal pipes and to an apparatus for carrying out the process.
2. Discussion of the Prior Art
At the present time, the pipes are produced in sections of a length of 6 to 15 m and are equipped with an inner covering which is corrosion-proof or abrasion-proof according to the fluids conveyed.
The term "fluids" is intended to refer in the most general way to petroleum liquids, water, chemical solutions and lyes and suspensions in water of solids, such as small coal or various slurries.
Pipes designed for conveying these fluids usually receive internal protection which, depending on the circumstances and the degree of development in the art, comprises centrifuged cement, bitumens, coverings based on thermoplastic resins, coverings based on thermosetting resins or coverings based on elastomers.
With all of these coverings, because of their tendency to cracking, their fusibility or their tendency to pyrolysis at the temperature used for joining the pipes, according to circumstances, it is necessary to reserve, at the ends of the pipes, an uncovered part, of which the dimension in the axial direction of the pipes, i.e. in the direction of the generatrices of the pipes, can vary between half and double the diameter, according to the pipe diameter, the wall thickness of the pipes and the type of covering.
These uncovered parts are therefor subject to corrosion or abrasion which is intense because it is concentrated, and which necessitates expensive replacement of the pipes after more or less long periods of time.
Other solutions resort to metal flanges equipped with elastomeric lips bearing on the inner covering which, in this case, is provided up to the ends of the pipe sections without any reserve. The disadvantage of such solutions is that the elastomeric lips do not withstand the high pressures and have to be reserved for low pressures only (below 3500 kPa).
Another solution involves using stainless-steel connecting rings. This is a solution which still entails a high outlay and which can even result in the use of complex techniques when the inner covering of the pipes is thick, for example if an especially high abrasion resistance is sought. A shoulder allowing for the thickness of the covering then has to be provided, and good sealing is difficult to obtain in this case. Moreover, whatever the composition of the heat-sensitive covering with which the inner wall of the pipes is equipped, in all cases there is a difference in composition between the adjacent coverings and therefore a difference in their abrasion resistance, thus resulting in a difference in the wear of the surfaces of the coverings and, in the long term, a variation in the inside diameter of the pipes near the junction.